The present invention relates to a starch ester wherein reactive hydroxyl groups in the same starch molecule have been replaced by acyl groups and groups derived therefrom (hereinafter referred to collectively as xe2x80x9cacyl groupsxe2x80x9d). Further, the present invention relates to a starch ester preferable as a base polymer in biodegradable starch-based plastic processed articles.
As used herein, the plastic processed articles include molded processed articles and modified processed articles molding-processed or modified as a whole or partially with a plastic composition, and the molding processing includes injection molding, extrusion molding, inflation, T-die extrusion, calendering, compression molding (press molding), transfer molding, casting, laminating, vacuum molding, blow molding, foam molding, coating, flow casting, heat fusion, stretching etc. (see 5th Handbook of Chemistry, Applied Chemistry I, p. 773, Table 10.22, Mar. 15, 1995, compiled by the Japanese Society of Chemistry and published by Maruzen). Accordingly, the molded processed articles include not only molded articles having a three-dimensional form but also films, sheets, coated papers, etc. Further, the modified processed articles include not only papers, processed papers and non-woven fabrics, but also articles produced by adding starch-substituted derivatives as modifiers to papers, non-woven fabrics, etc., made of natural materials.
The basic method of modifying starch, associated with the present invention, is esterification (acylation), and the starch ester produced by this reaction has been known as low-substituted starch (starch ester) esterified in anaqueous reaction system (xe2x80x9cStarch Science Handbookxe2x80x9d, K. K. Asakura Shoten, p. 550).
With respect to high-substituted starch ester (esterified starch), a method of reacting an acid anhydride in pyridine by use of dimethyl amino pyridine or an alkali metal as a catalyst (xe2x80x9cStarch Chemistry and Technologyxe2x80x9d authored by Whistler, published by Academic Press, pp. 332-336), a method of reacting an acid anhydride at a high temperature of 100xc2x0 C. or more by use of an aqueous solution of an alkali metal hydroxide as a catalyst (Japanese National Publication No. 508,185/1993, and p. 73 in the March issue of Die Starke, 1972), and a method of reaction in a non-aqueous organic solvent (Japanese Patent Laid-Open No. 188,601/1996) are known.
With an increasing awareness of environmental problems in recent years, starch esters produced by the, methods described above have been used in various biodegradable plastic materials. However, these materials, whether used alone for forming molded articles or films or in combination with various synthetic resins, require a general-purpose plasticizer (phthalate type or fatty ester type) in order to achieve workability (for example, injection workability, extrusion workability, stretchability, etc.) at the same levels as ordinary thermoplastic plastics (thermoplastic resin).
Even if produced using the plasticizer, products such as injection-molded articles hardly achieve impact strength at the same levels as with impact strength polystyrene (high impact polystyrene). It has also been difficult to achieve molded articles having an impact resistance of 1.8 kgfxc2x7cm/cm (17.64 J/m) or more in terms of Izod impact strength (ASTM D256: xe2x88x9223xc2x0 C.).
Further, products such as inflation films have hardly achieved stretchability (tensile elongation) as good as that of polyethylene.
In particular, these tendencies become significant as the ratio of the starch ester in the plastic composition (plastic material) to be molded is increased.
Even if a biodegradable resin (biodegradable polymer) other than the starch ester is mixed in an attempt to improve the impact strength or tensile elongation of the starch ester, the desired improvement effects cannot be attained unless the content of the biodegradable resin is made to be higher than the content of the starch ester. As a result, such products cannot truly be said to be biodegradable plastics that are based on a starch ester.
Further, the phthalate or fatty ester type plasticizer described above is suspected of being a physiologically disturbing substance, which adversely affects vegetables, foods, and the growth of animals. Accordingly, one should avoid adding the plasticizer described above to biodegradable plastics that are to be disposed of in landfills, etc.
In view of the foregoing, an object of the present invention is to provide a starch ester which can be used as a thermoplastic material capable of being thermo-plasticized in the absence of a plasticizer or by using a small amount of a plasticizer.
Another object of the present invention is to provide a starch ester from which a thermoplastic plastic material having superior impact strength and tensile elongation can be easily prepared.
The present inventors made extensive study regarding the development of safe biodegradable plastics in the absence of a plasticizer or by using a small amount of a plasticizer, by use of starch which is an abundant raw material produced every year. The result of these studies is the novel starch ester having the constitution described below.
The present invention relates to a starch ester wherein reactive hydroxyl groups in the same starch molecule have been replaced by a C2-4 acyl group (referred to hereinafter as xe2x80x9cshort-chain acyl groupxe2x80x9d) and a C6-18 acyl group (referred to hereinafter as xe2x80x9clong-chain acyl groupxe2x80x9d), and the extent of substitution by the short- and long-chain acyl groups are regulated so as to make the starch ester thermo-plasticized and moldable even in the absence of a plasticizer.
From the viewpoint of workability, the starch ester as used herein is preferably one having a glass transition point by differential thermal analysis (JIS K 7121: referred to hereinafter as xe2x80x9cglass transition pointxe2x80x9d) of 140xc2x0 C. or less, preferably 130xc2x0 C. or less. The lower limit of the glass transition point shall be usually 80xc2x0 C., preferably 100xc2x0 C.
To easily attain each characteristic, a starch ester having the workability or showing the glass transition point as described above is preferably one wherein the degree of substitution by the long-chain acyl group is from 0.06 to 2.0, the degree of substitution by the short-chain acyl group is from 0.9 to 2.7, and the degree of substitution by the total acyl groups is from 1.5 to 2.95, more preferably one wherein the degree of substitution by the long-chain acyl group is from 0.1 to 1.6, the degree of substitution by the short-chain acyl group is from 1.2 to 2.1, and the degree of substitution by the total acyl groups is from 1.7 to 2.9.
The starch ester of the present invention can also be used in a starch ester-based polymer alloy by incorporating the starch ester with a biodegradable resin. Polycaprolactone, polylactic acid or cellulose acetate can be used particularly preferably as the biodegradable resin.
Further, the starch ester of the present invention can be formed into a molded processed article which has been molded and processed as a whole or partially with said starch ester or a polymer alloy having said starch ester incorporated with a biodegradable resin.
The molded processed article can be formed into an injection-molded article showing a degree of water absorption (after immersion in tap water at 23xc2x0 C. for 24 hours) of 0.5% or less and an Izod impact strength of 1.8 kgfxc2x7cm/cm, or into a film having a film thickness of 100 xcexcm or less and a tensile elongation (JIS K 6301) of 200% or more.
Further, the starch ester of the present invention can be formed into a plastic processed article which has been molded and processed, or modified, as a whole or partially with a plastic composition comprising an organic or inorganic reinforcing filler added to said starch ester or to a polymer alloy which is an admixture of the starch ester and a biodegradable resin.
Hereinafter, the means of the present invention is described in detail. The blend unit is expressed on a weight basis unless otherwise specified. In the following description, Cn in round brackets after each compound indicates that the number of carbons in acyl groups in the compound is n.
As used herein, the degree of substitution (DS) is the average number of reactive hydroxyl groups (that is, 3 hydroxyl groups at the 2-, 3- and 6- or 4-positions) replaced by substituent groups per glucose residue in a starch derivative, and when DS is 3, the degree of masking (substitution percent) of the reactive hydroxyl groups is 100%.
As a result of intensive study for solving the problem described above, the present inventors found that it is essential to confer thermoplasticity on starch itself in a practical temperature range in order to solve the problem, and it is important therefor to bind long-chain hydrocarbon-containing groups such as long-chain alkyl groups, cycloalkyl groups, alkylene groups and aryl groups, along with short-chain hydrocarbon-containing groups such as short-chain alkyl groups, cycloalkyl groupa, alkylene groups and aryl groups, to the same starch molecule. By so doing, the present invention arrived at the novel starch ester with the constitution described below.
Said starch ester is conceptually shown in the structural formula: 
wherein. R1 is a C2-4 short-chain acyl group, and R2 is a C6-18 long-chain acyl group.
Although the process for producing said starch ester is not particularly limited, the starch ester can be easily produced by a process constituted as follows (see Japanese Patent Laid-Open No. 188,601/1996 (Japanese Patent No. 2,579,843)): xe2x80x9cA process for producing a starch ester by using a vinyl ester as an esterification reagent wherein a vinyl ester having a C2-18 ester, group is reacted with starch in a non-aqueous organic solvent using an esterification catalyst.xe2x80x9d
That is, the biodegradable starch ester of the present invention can be easily synthesized through acylation (esterification) in a non-aqueous organic solvent by replacing reactive hydroxide groups in the same starch molecule by long-chain acyl groups derived from vinyl compounds, acid anhydrides acid halides, alkyl ketene dimers or lactones, along with short-chain acyl groups from the same compounds.
By such means, the present inventors found for the first time that:
1) By these reactive groups, it is possible to synthesize starch esters exhibiting thermoplasticity during heating in the absence of a plasticizer or by using a small amount of a plasticizer;
2) These starch esters show a significantly higher miscibility with biodegradable resins other than said starch esters than that of existing highly modified starch esters (prepared by the known method described above); and
3) Molded processed articles formed as a whole or partially from a plastic composition based on said starch esters have impact resistance similar to that of impact-resistant (high impact) polystyrene.
As the starting starch for the starch ester of the present invention, (1) unmodified starch from on the ground (soil), such as corn starch, high amylose starch, wheat starch and rice starch, (2) unmodified starch in the ground, such as potato starch and tapioca starch, and (3) starch esters prepared by subjecting the above-described starches to low-degree esterification, etherification, oxidation, acid treatment, or conversion into dextrin; these starches can be used alone or in a combination thereof.
The acylation (esterification) reagent used for introducing C6-18 long-chain acyl groups onto reactive hydroxyl groups by substitution reaction includes one or more members selected from alkyl ketene dimers, cyclic esters (caprolactones), acid anhydrides, acid halides and vinyl compounds having esterification (acylation) reactive sites having C5-17 long-chain hydrocarbon groups bound to carbonyl groups (number of carbons in one molecule of the reagent: 6 to 18).
The long-chain hydrocarbon groups described above include an alkyl group, a cycloalkyl group, an alkylene group and an aryl group as well as groups derived therefrom. The derived groups include an aryl alkyl group (aralkyl), alkyl aryl group (alkaryl), and alkoxy alkyl group. The long-chain hydrocarbon groups also include active hydrogen groups such as a hydroxy alkyl group and an aminoalkyl group, insofar as the effect of the present invention is not adversely affected.
Among these compounds, esterification reagents having C8-14 esterification reaction sites are preferable for reaction efficiency and handling.
The alkyl ketene dimers are constituted of a combination of various alkyl groups, as represented by the formula: 
wherein R is a C5-17 alkyl group, an alkylene group, an aryl group, or a group derived therefrom.
As the cyclic esters (caprolactones), xcex5-caprolactone (C6), xcex3-caprylolactone (C8), xcex3-laurolactone (C12) and xcex3-stearolactone (C18), as well as large cyclic lactones represented by the formula (CH2)nCOO wherein n is an integer from 5to 17; these can be used singly or in combination thereof.
As the acid anhydrides and acid halides, anhydrides and halides of caprylic acid (C8), lauric acid (C12), pal mitic acid (C16), stearic acid (C18), oleic acid (C18), etc., can be used.
As the vinyl compounds, it is possible to use saturated or unsaturated vinyl aliphatic carboxylates such as vinyl caprylate (C8), vinyl laurate (C12), vinyl palmitate (C16), vinyl stearate (C18) and vinyl oleate (C18), and branched saturated vinyl aliphatic carboxylates represented by the following structural formula: 
wherein all R1, R2 and R3 are alkyl groups, and the number of carbons in these groups in total is from 4 to 16.
The non-aqueous polar organic solvent is one capable of dissolving the starting starch, and specifically, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), pyridine, etc., can be used alone or in a combination thereof, or these can be used as a mixture with another organic solvent.
The esterification (acylation) catalyst used includes one or more members selected from the following respective groups: (1) hydroxides, mineral acid salts, carbonates, organic compounds or alkali metal alkoxides of metals up to the 5th period in the Periodic Table, (2) organic-interlayer transfer catalysts, and (3) amino compounds. Among these, (1) is desirable from the viewpoint of reaction efficiency and catalyst costs.
Examples of the catalysts are as follows:
(1) Alkali metal hydroxides such as sodium hydroxide, potassium hydroxide and lithium hydroxide; alkali metal organic acid salts such as sodium acetate and sodium p-toluene sulfonate; alkaline earth metal hydroxides such as barium hydroxide and calcium hydroxide, alkaline earth metal organic acid salts such as calcium acetate, calcium propionate and barium p-toluene sulfonate; inorganic acid salts such as sodium phosphate, calcium phosphate, sodium hydrogen sulfite, sodium carbonate, sodium hydrogen carbonate, potassium carbonate, potassium hydrogen carbonate, potassium sulfate, sodium aluminate and potassium zincate; and amphoteric metal hydroxides such as aluminum hydroxide and zinc hydroxide; and
(2) Amino compounds such as dimethyl amino pyridine and diethyl amino acetic acid,
as well as quaternary ammonium compounds such as N-trimethyl-N-propyl ammonium chloride and N-tetraethyl ammonium chloride. The timing and method of using these catalysts are not particularly limited.
The acylation (esterification) reagent used for introducing C2-4 short-chain acyl groups onto reactive hydroxyl groups by substitution reaction includes one or more members selected from alkyl ketene dimers, cyclic esters (caprolactones), acid anhydrides, acid halides and vinyl compounds having esterification (acylation) reaction sites having C1-3 short-chain hydrocarbon groups bound to carbonyl groups (that is, the number of carbons in each reagent molecule is from 2 to 4).
Specifically, the following compounds can be mentioned. Among these compounds, those acylation reagents having C2-3 esterification reaction sites are preferable for reaction efficiency, microbial degradation and handling.
As the cyclic esters (caprolactones), xcex3-butyrolactone (C3) and xcex2-propiolactone (C3) can be used singly or in combination thereof.
As the acid anhydrides and acid halides, anhydrides and halides of acetic acid (C2), propionic acid (C3), butanoic acid (C4), etc., can be used singly or in combination thereof.
As the vinyl compounds, vinyl acetate (C2), vinyl propionate (C3), vinyl butanoate (C4), vinyl acrylate (C3), vinyl isocrotonate (C4), etc., can be used.
Although the reaction temperature condition in the present invention is not particularly limited, the reaction temperature shall be usually 30xc2x0 C. to 200xc2x0 C., preferably 40xc2x0 C. to 150xc2x0 C. For almost all compounds, it will not be necessary to change these reaction temperatures.
With respect to the degree of substitution (DS) on the starch ester, the compatibility of the starch ester with a biodegradable resin as an object of the present invention is affected by, the length of the long-chain acyl group. However, with the long-chain acyl group having the maximum number of carbons given, it is difficult to confer the desired characteristics on starch insofar as the degree of substitution (DS) by said acyl group is 0.05 or less (or the degree of masking of reactive hydroxyl groups is 2% or less). As the acyl group having the maximum number of carbons, the acyl group containing 19 or more carbon atoms is not practical because the reaction efficiency is extremely lowered thereby.
Usually, the DS by the long-chain acyl group is from 0.06 to 2.0 (degree of masking: 2% to 67%), the DS by the short-chain acyl group is from 0.9 to 2.7 (degree of masking: 30% to 90%), and the DS by the total acyl groups is from 1.5 to 2.95 (degree of masking: 50% to 98%).
Between the starch ester wherein the degree of substitution by the long-chain acyl group is minimal and the degree of substitution by the short-chain acyl group is maximal, and the starch ester wherein the degree of substitution by the long-chain acyl group is maximal and the degree of substitution by the short-chain acyl group is minimal, there is no extreme difference in the compatibility thereof with a biodegradable resin nor in the mechanical.physical properties thereof. To achieve the same level of thermoplasticity in the absence of a plasticizer, the degree of substitution by the long-chain acyl group may be decreased as the number of carbons in said acyl group is increased.
Accordingly, the above-described numerical range has no particular critical importance, and the present invention can be carried out even in the vicinity of the above-described range.
Preferably, the DS by the long-chain acyl group is from 0.1 to 1.6 (degree of masking: 3% to 53%), the DS by the short-chain acyl group is from 1.2 to 2.1 (degree of masking: 40% to 70%), and the DS by the total acyl groups is from 2.0 to 2.9 (degree of masking: 67% to 97%).
The reason that the number of carbons in the short-chain acyl group shall be 4 or less is based on the experimental result that in the present invention, there is no difference in reaction efficiency among C2-4 short-chain acyl groups.
With respect to the glass transition point (JIS K 7121) of the starch, the miscibility of the starch with the biodegradable resin becomes gradually poor as the transition point (transition temperature) is increased. Usually, the glass transition point shall be 140xc2x0 C. or less, preferably 80xc2x0 C. to 130xc2x0 C. This is because if the glass transition point is higher than 140xc2x0 C., miscibility becomes poor in the absence of a plasticizer. If a plasticizer is used, the starch ester is rendered miscible even at a temperature of higher than 140xc2x0 C. and in a smaller amount of the plasticizer than is conventional.
Hereinafter, the biodegradable polymer (biodegradable resin) incorporated with the starch ester of the present invention to form a polymer alloy is described.
In the present invention, the term xe2x80x9cincorporatedxe2x80x9d means that two or more materials are admixed with xe2x80x9ccompatibilityxe2x80x9d, and the term xe2x80x9ccompatibilityxe2x80x9d refers to the state of two or more materials in which they are uniformly and mutually dispersed, including not only the state attained by mixing two or more materials having mutual miscibility, but also the state where two or more materials, although being mutually xe2x80x9cimmisciblexe2x80x9d, are uniformly dispersed.
As can also be easily judged from the above-described glass transition temperature (glass transition point), the starch ester of the present invention can be thermoplasticized without using an oily plasticizer. Further, the starch ester of the present inventions does not require any plasticizer for blending thereof with the existent biodegradable resin, and the compatibility thereof is significantly improved as compared with that of starch esters produced in the prior art, such as high-substituted acetylated starch (acetate starch).
As the biodegradable polymer described above, the following polymers of a natural type (mainly cellulose type) or of a synthetic type (polymerized type) can be preferably used.
That is, the polymers of cellulose type include cellulose acetate, hydroxyethyl cellulose, propyl cellulose, hydroxybutyl cellulose, etc.
The polymers of polymerized type include:
(1) biodegradable polyesters or polyamides such as polycaprolactone (PCL), polylactic acid (PLA), polyadipate, polyhydroxy butyrate (polyhydroxy alkanoates), polyhydroxy butyrate valerate (PHB/V) and succinic acid-1,4-butanediol polymers;
(2) polyalkylene oxides such as polyethylene oxide and polypropylene oxide; and
(3) vinyl polymers such as polyvinyl alcohol, modified polyvinyl alcohol, polyacrylamide-based resin, polycarbonate-based resin, polyurethane-based resin, polyvinyl acetate, polyvinyl carbazole, polyacrylate, and ethylene-vinyl acetate copolymers.
When the starch ester of the present invention or the polymer alloy described above is used as a base polymer to prepare a plastic material (starch ester-based composition), the following various fillers can be used as fillers that are used together with other auxiliary materials.
The form of the fillers can be selected arbitrarily as necessary from powder, granules, plates, cylinders, fibers and needles.
The inorganic fillers include talc, titanium oxide, clay, chalk, limestone, calcium carbonate, mica, glass, silica and various silica salts, diatomaceous earth, wall austenite, various magnesium salts, various manganese salts etc.
The organic fillers include starch and starch derivatives, cellulose and derivatives thereof, wood powder, pulp, pecan fibers, cotton powder, corn husk, cotton linter, wood fibers, bagasse, etc.
The synthetic fillers include glass fibers, urea polymers, ceramics, etc.